THE VARIATION, RACES AND MIGRATIONS OF THE MACKEREL. 273 



Table VIII., sliowing mean nuniber of first dorsal finrays for males 



and females of different sizes. 



This table shows two things : — 



1. That a reduction in the number of recognisable finraj^s with in- 

 creasing size is a general phenomenon, common to mackerel from 

 Irish and English seas alike : and 



2. That there are no sexual peculiarities at any size in regard to the 

 number of finrays in the first dorsal fin. 



The first of these conclusions is obvious enough. The second con- 

 clusion is founded on the practical identity of the mean values for 

 Irish males and females at corresponding sizes, and on the inconsistency 

 in kind of the differences which exist between the males and females 

 of the Channel and North Sea. The mean for the females in the first 

 compartment exceeds that for the males by 0"10, while in the second 

 compartment the mean for the males exceeds that for the females by 

 01 1. Whatever may be the explanation of these deviations between 

 the male and female values, it is clearly not due to the existence of any 

 secondary sexual peculiarities in the number of finrays. 



The questions naturally arise, at what period in the life of a mackerel 

 does the process of reduction begin, and what is the cause of the 

 reduction ? 



The first question can only be answered after the examination of 

 large numbers of young mackerel. The material at my disposal is 

 at present too limited for me to go minutely into the matter, but is 

 sufficient to show that the reduction in the number of rays begins 

 before the attainment of a length of 10 inches. The mean number of 

 finrays in 127 Plymouth mackerel above 12i inches in length is 12*02; 

 the mean in 129 small mackerel from the same locality, all of which 

 were below ]0| inches in length, i.s 12"49. The frequency of the 

 different values is shown in the following table : — 



